[DRE-maint] ruby-doorkeeper is marked for autoremoval from testing

[Debian testing autoremoval watch]

ruby-doorkeeper 4.2.0-3 is marked for autoremoval from testing on 2017-09-02

It is affected by these RC bugs:
869057: ruby-doorkeeper: FTBFS: ERROR: Test "ruby2.3" failed: Invalid gemspec in [doorkeeper.gemspec]: No such file or directory - git